Legacy read receipt request: Return-Receipt-To

app/src/main/java/eu/faircode/email/EmailService.java

@@ -1337,7 +1337,7 @@ public class EmailService implements AutoCloseable {
                 " reuse=" + reuse +
                 " delay=" + delay);
 
-        if (keepAlive) {
+        if (keepAlive || BuildConfig.DEBUG) {
             Log.e("Socket keep-alive=" + keepAlive);
             socket.setKeepAlive(false); // sets SOL_SOCKET/SO_KEEPALIVE
         }

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -550,6 +550,8 @@ public class MessageHelper {
                     // Read receipt
                     imessage.addHeader("Disposition-Notification-To", to);
                     imessage.addHeader("Read-Receipt-To", to);
+                    if (receipt_legacy)
+                        imessage.addHeader("Return-Receipt-To", to);
                     imessage.addHeader("X-Confirm-Reading-To", to);
                 }
             }